Flexure Pivot Tilting Pad Bearings Research Articles

Experimental analysis of the thermal unbalance effect of a flexible rotor supported by a flexure pivot tilting pad bearing

The Morton effect is a thermally induced increase of synchronous vibrations amplitudes of rotors. The shearing of the fluid film in a journal bearing produces an asymmetric heating of the rotor which causes its thermal bending and increases the unbalance. The vector of synchronous vibrations describes a spiral with slowly increasing amplitude and phase change. The thermal bending is also enlightened by a hysteresis effect of the synchronous vibrations during start-up and coast-down. The purpose of this study is to highlight experimentally the signature of Morton effect in a flexible rotor supported by a flexure pivot tilting pad journal bearing. A dedicated test rig has been built. It consists of a flexible rotor guided at one end by a ball bearing and at the other end by a flexure pivot tilting pad bearing. The tests consist of controlled start-ups to a speed close to the critical speed of the first rotor flexible mode, followed by controlled coast-downs. They show a hysteresis of the amplitudes and phases of the synchronous vibrations. Other tests performed at constant speeds show amplitude and phase variations corresponding to spiral vibrations. The results are explained by the non-uniform distribution of the rotor temperature.

Bump-Type Foil Bearings and Flexure Pivot Tilting Pad Bearings for Oil-Free Automotive Turbochargers: Highlights in Rotordynamic Performance

Oil-free turbochargers (TCs) require gas bearings in compact units of enhanced rotordynamic stability, mechanical efficiency, and improved reliability with reduced maintenance costs compared with oil-lubricated bearings. Implementation of gas bearings into automotive TCs requires careful thermal management with accurate measurements verifying model predictions. Gas foil bearings (GFBs) are customarily used in oil-free microturbomachinery because of their distinct advantages including tolerance to shaft misalignment and centrifugal/thermal growth, and large damping and load capacity compared with rigid surface gas bearings. Flexure pivot tilting pad bearings (FPTPBs) are widely used in high-performance turbomachinery since they offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability. The paper details the rotordynamic performance and temperature characteristics of two prototype oil-free TCs; one supported on foil journal and thrust bearings and the other one is supported on FPTP journal bearings and foil thrust bearings of identical sizes (outer diameter (OD) and inner diameter (ID)) with the same aerodynamic components. The tests of the oil-free TCs, each consisting of a hollow rotor (∼0.4 kg and ∼23 mm in OD at the bearing locations), are performed for various imbalances in noise, vibration, and harshness (NVH; i.e., cold air driven rotordynamics rig) and gas stand test facilities up to 130 krpm. No forced cooling air flow streams are supplied to the test bearings and rotor. The measurements demonstrate the stable performance of the rotor–gas bearing systems in an ambient NVH test cell with cold forced air into the turbine inlet. Post-test inspection of the test FPTPGBs after the hot gas stand tests evidences seizure of the hottest bearing, thereby revealing a notable reduction in bearing clearance as the rotor temperature increases. The compliant FPTPGBs offer a sound solution for stable rotor support only at an ambient temperature condition while demonstrating less tolerance for shaft growth, centrifugal, and thermal, beyond its clearance. The current measurements give confidence in the present GFB technology for ready application into automotive TCs for passenger car and commercial vehicle applications with increased reliability.

Hybrid Gas Bearings With Controlled Supply Pressure to Eliminate Rotor Vibrations While Crossing System Critical Speeds

Microturbomachinery implements gas bearings in compact units of enhanced mechanical reliability. Gas bearings, however, have little damping and wear quickly during transient rub events. Flexure pivot tilting pad bearings offer little or no cross-coupled stiffnesses with enhanced rotordynamic stability; and when modified for hydrostatic pressurization, demonstrate superior rotordynamic performance over other bearing types. External pressurization stiffens gas bearings thus increasing system critical speeds, albeit reducing system damping. Most importantly, measurements demonstrate that external pressurization is not needed for rotor supercritical speed operation. In practice, the supply pressure could be shut off at high rotor speeds with substantial gains in efficiency. This paper introduces a simple strategy, employing an inexpensive air pressure regulator to control the supply pressure into the hybrid bearings, to reduce or even eliminate high amplitudes of rotor motion while crossing the system critical speeds. Rotor speed coast-down tests with the pressure controller demonstrate the effectiveness of the proposed approach. A simple on-off supply pressure control, i.e., a sudden increase in pressure while approaching a critical speed, is the best since it changes abruptly the bearing stiffness coefficients and moves the system critical speed to a higher speed. A rotordynamic analysis integrating predicted bearing force coefficients forwards critical speeds in agreement with the test results. Predicted rotor responses for the controlled supply conditions show an excellent correlation with measured data. The experiments validate the predictive tools and demonstrate the controllable rotordynamic characteristics of flexure pivot hybrid gas bearings.

Hybrid Flexure Pivot-Tilting Pad Gas Bearings: Analysis and Experimental Validation

Gas film bearings offer unique advantages enabling successful deployment of high-speed microturbomachinery. Current applications encompass micro power generators, air cycle machines, and turbo expanders. Mechanically complex gas foil bearings are in use; however, their excessive cost and lack of calibrated predictive tools deter their application to mass-produced oil-free turbochargers, for example. The present investigation advances the analysis and experimental validation of hybrid gas bearings with static and dynamic force characteristics desirable in high-speed turbomachinery. These characteristics are adequate load support, good stiffness and damping coefficients, low friction and wear during rotor startup and shutdown, and most importantly, enhanced rotordynamic stability at the operating speed. Hybrid (hydrostatic/hydrodynamic) flexure pivot-tilting pad bearings demonstrate superior static and dynamic forced performance than other geometries as evidenced in a high-speed rotor-bearing test rig. A computational model including the effects of external pressurization predicts the rotordynamic coefficients of the test bearings and shows good correlation with measured force coefficients, thus lending credence to the predictive model. In general, direct stiffnesses increase with operating speed and external pressurization, whereas damping coefficients show an opposite behavior. Predicted mass flow rates validate the inherent restrictor-type orifice flow model for external pressurization. Measured coast-down rotor speeds demonstrate very low-friction operation with large system time constants. Estimated drag torques from the gas bearings indirectly validate the recorded system time constant.

Rotordynamic Coefficients Measurements Versus Predictions for a High-Speed Flexure-Pivot Tilting-Pad Bearing (Load-Between-Pad Configuration)

Experimental dynamic force coefficients are presented for a four pad flexure-pivot tilting-pad bearing in load-between-pad configuration for a range of rotor speeds and bearing unit loadings. Measured dynamic coefficients have been compared to theoretical predictions using an isothermal analysis for a bulk-flow Navier-Stokes (NS) model. Predictions from two models—the Reynolds equation and a bulk-flow NS equation models are compared to experimental, complex dynamic stiffness coefficients (direct and cross-coupled) and show the following results: (i) The real part of the direct dynamic-stiffness coefficients is strongly frequency dependent because of pad inertia, support flexibility, and the effect of fluid inertia. This frequency dependency can be accurately modeled for by adding a direct added-mass term to the conventional stiffness/damping matrix model. (ii) Both models underpredict the identified added-mass coefficient (∼32kg), but the bulk-flow NS equation predictions are modestly closer. (iii) The imaginary part of the direct dynamic-stiffness coefficient (leading to direct damping) is a largely linear function of excitation frequency, leading to a constant (frequency-independent) direct damping model. (iv) The real part of the cross-coupled dynamic-stiffness coefficients shows larger destabilizing forces than predicted by either model. The frequency dependency that is accounted for by the added mass coefficient is predicted by the models and arises (in the models) primarily because of the reduction in degrees of freedom from the initial 12 degrees (four pads times three degrees of freedom) to the two-rotor degrees of freedom. For the bearing and condition tested, pad and fluid inertia are secondary considerations out to running speed. The direct stiffness and damping coefficients increase with load, while increasing and decreasing with rotor speed, respectively. As expected, a small whirl frequency ratio (WFR) was found of about 0.15, and it decreases with increasing load and increases with increasing speed. The two model predictions for WFR are comparable and both underpredict the measured WFR values. Rotors supported by either conventional tilting-pad bearings or flexure-pivot tilting-pad (FPTP) bearings are customarily modeled by frequency-dependent stiffness and damping matrices, necessitating an iterative calculation for rotordynamic stability. For the bearing tested and the load conditions examined, the present results show that adding a constant mass matrix to the FPTP bearing model produces an accurate frequency-independent model that eliminates the need for iterative rotordynamic stability calculations. Different results may be obtained for conventional tilting-pad bearings (or this bearing at higher load conditions).

Measurements of the Static Load (On Pad) Performance and Pad Temperatures in a Flexure-Pivot Tilting-Pad Bearing

Measurements of the bearing displacements and pad edge temperatures in a four-shoe flexure-pivot tilting-pad bearing under static loading toward a pad are presented. The test rig consists of a vertical rigid journal and shaft supported on precision ball bearings and driven by a variable speed motor. The housing is suspended by cables and holds the flexure-pivot bearing with four 80° arc length pads, 50% offset and null preload. The length, diameter, and nominal radial clearance of the bearing are equal to 46 mm, 127 mm, and 0.178 mm, respectively, with a pad flexural rotational stiffness equal to 1125 N-m/rad. Steady-state tests with an ISO VG 22 oil are conducted at three rotational speeds (1800, 3000, and 4500 rpm) with loads applied to the bearing with a simple jack and load cell mechanism. Measurements include the bearing displacements in two orthogonal directions, oil flow rate, lubricant inlet and discharge temperatures, and the leading and trailing edge pad temperatures for each pad. The measurements show the bearing displacements to be larger in the direction of the applied loan than in the orthogonal direction with an attitude angle of approximately 30°. This characteristic bearing operation is due to the relatively large size of the bearing clearance and the pad rotational stiffness. The test bearing does not show any subsynchronous whirl (hydrodynamic instability) including operation at the null bad condition. The pad temperatures follow well-known trends with the highest magnitude at the trailing edge of the loaded pad. A reduction of the lubricant viscosity in the tests at the largest speed (4500 rpm) produced a drop in the bearing load capacity because of the increase of the lubricant and pad temperatures. Overall, the experimental performance curve of bearing eccentricity versus Sommerfeld number agrees well with numerical predictions based on an effective viscosity method. Presented as a Society of Tribologists and Lubrication Engineers paper at the World Tribology Congress in London, United Kingdom, September 8–12, 1997

Measurements of Static Loading Versus Eccentricity in a Flexure-Pivot Tilting Pad Journal Bearing

An experimental investigation examining the static loading characteristics of a four-pad, flexure-pivot tilting pad bearing is presented. Tests are conducted on a Fluid Film Bearing Element Test Rig for journal speeds ranging from 1800 to 4500 rpm, and applied static loads between pads to 1400 N. Results obtained from measurements of bearing eccentricities and bearing pad temperatures were compared to numerical predictions. Bearing power losses were also estimated using a simple thermal model for the bearing assembly. Comparisons between theory and experimental results indicate good correlation for measured eccentricities over the tested range of applied static loads and journal speeds. Negligible displacements in the direction orthogonal to the applied load verified the expected behavior of the test bearing over the range of operating conditions. Thermal analysis of the hearing system lead to good prediction of the bearing power losses, and indicated that the majority of the mechanical energy is transferred via convection.